U.S. patent number 6,264,812 [Application Number 08/559,345] was granted by the patent office on 2001-07-24 for method and apparatus for generating a plasma.
This patent grant is currently assigned to Applied Materials, Inc.. Invention is credited to John Forster, Ivo J. Raaijmakers, Bradley O. Stimson.
United States Patent |
6,264,812 |
Raaijmakers , et
al. |
July 24, 2001 |
Method and apparatus for generating a plasma
Abstract
A method and apparatus for generating a plasma by inductively
coupling electromagnetic energy into the plasma. In one embodiment,
first and second antenna coils are disposed about the circumference
of the plasma containment area. The first and second antenna coils
are relatively spaced along the longitudinal axis of the plasma
containment area. A current is generated in the first and second
antenna coils. A phase shift regulating network establishes a
difference between the phase of the current in the first antenna
and the phase of the current in the second antenna. The phase
difference corresponds to the phase difference required to launch a
helicon wave in the plasma. In a second embodiment, a chamber
shield is made of a conductive material and is coupled to the RF
source such that the shield functions as an RF antenna. The shield
may be coupled in series to a coil surrounding the shield to
increase the resultant flux density.
Inventors: |
Raaijmakers; Ivo J. (Phoenix,
AZ), Stimson; Bradley O. (San Jose, CA), Forster;
John (San Francisco, CA) |
Assignee: |
Applied Materials, Inc. (Santa
Clara, CA)
|
Family
ID: |
24233249 |
Appl.
No.: |
08/559,345 |
Filed: |
November 15, 1995 |
Current U.S.
Class: |
204/298.06;
118/723AN; 118/723E; 118/723I; 118/723IR; 204/298.11; 204/298.16;
204/298.19; 204/298.2; 156/345.49 |
Current CPC
Class: |
H01J
37/321 (20130101); H01J 37/32477 (20130101); H01J
37/32165 (20130101) |
Current International
Class: |
H01J
37/32 (20060101); C23C 014/34 () |
Field of
Search: |
;156/345
;204/298.06,298.08,298.11,298.16,298.19,298.2
;118/723MW,723ME,723MR,723MA,723AN,723E,723ER,723I,723IR |
References Cited
[Referenced By]
U.S. Patent Documents
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|
Primary Examiner: McDonald; Rodney
Attorney, Agent or Firm: Konrad, Raynes & Victor LLP
Claims
What is claimed is:
1. An apparatus for launching a helicon wave in a magnetized plasma
within a semiconductor fabrication chamber by coupling
electromagnetic energy into the plasma, the apparatus
comprising:
a plasma chamber defining a plasma generation area having a
circumference and a longitudinal axis;
a first antenna forming a coil about the circumference of the
plasma generation area;
a second antenna forming a coil about the circumference of the
plasma generation area;
the first and second antenna coils being relatively spaced along
the longitudinal axis of the plasma generation area and
electrically isolated from each other;
means for generating a current in the first antenna coil and in the
second antenna coil, the current in the first antenna coil having a
phase and the current in the second antenna coil having a
phase;
means for establishing a difference between the phase of the
current in the first antenna coil and the phase of the current in
the second antenna coil, the difference between the phase of the
current in the first antenna coil and the phase of the current in
the second antenna coil corresponding to a phase difference
required to launch a helicon wave with a desired wavelength in the
plasma; and
means for establishing a substantially uniform axial magnetic field
in the plasma.
2. The apparatus of claim 1 wherein at least one of the first and
second antenna comprises a shield for protecting the plasma chamber
from metal deposition.
3. The apparatus of claim 1 wherein the phase difference
establishing means has means for varying the difference between the
phase of the current in the first antenna coil and the phase of the
current in the second antenna coil.
4. The apparatus of claim 1 wherein at least one of the first and
second antenna coils comprises a first conductive shield positioned
within the chamber to protect at least a portion of the chamber
wall from deposition materials; and a coil electrically coupled in
series with the shield.
5. An apparatus for launching a helicon wave in a plasma by
coupling electromagnetic energy into the plasma, the apparatus
comprising:
a chamber having a chamber wall;
means for establishing an axial magnetic field within the
chamber;
a first conductive shield wall positioned within the chamber to
protect at least a portion of the chamber wall from deposition
materials;
a first RF source coupled to the first conductive shield wall to
radiate a first RF signal from the first conductive shield
wall;
a second conductive shield wall positioned within the chamber to
protect at least a portion of the chamber wall from deposition
materials; and
a second RF source coupled to the second conductive shield wall to
radiate a second RF signal from the second conductive shield
wall;
wherein the phase of said second RF signal differs from the phase
of said first RF signal such that a helicon wave with a desired
wavelength in the plasma is maintained.
6. The apparatus of claim 5 further comprising a first antenna coil
coupled in series with the first RF source and the first conductive
shield wall to radiate an RF signal in phase with the first RF
signal.
7. The apparatus of claim 6 further comprising a second antenna
coil coupled in series with the second RF source and the second
conductive shield wall to radiate an RF signal in phase with the
second RF signal.
8. The apparatus of claim 5 wherein the first and second RF signals
are inductively coupled with the plasma.
9. The apparatus of claim 5 wherein the chamber defines a
longitudinal axis and the first and second shield walls are
relatively spaced along the longitudinal axis of the chamber.
10. The apparatus of claim 5 wherein the first and second RF
signals radiated by the first and second shield walls,
respectively, have a predetermined phase difference required to
launch a helicon wave of desired wavelength .lambda..sub.z in the
plasma.
11. The apparatus of claim 5 further comprising a variable phase
shifter for shifting the phase between the first and second RF
signals radiated by the first and second shields, respectively.
12. The apparatus of claim 5 wherein the first and second shield
walls each have a generally cylindrically shaped surface facing the
interior of the chamber to intercept deposition materials.
13. The apparatus of claim 5 further comprising a generally disk
shaped chuck, wherein the second shield wall has a generally
annular-shaped surface circumferentially surrounding the chuck.
14. The apparatus of claim 5 further comprising a chuck and a cover
ring covering the perimeter of the chuck and the second shield
wall.
15. The apparatus of claim 5 further comprising a target adjacent
to the first shield and an insulator ring insulating the target
from the first shield wall.
16. The apparatus of claim 15, further comprising a magnetic field
source behind the target and a power supply to bias the target
negative with respect to the plasma.
17. The apparatus of claim 5 further comprising a chuck which is
negatively biased with respect to the plasma.
18. An apparatus for launching a helicon wave in a magnetized
plasma within a semiconductor processing system by coupling
electromagnetic energy into the plasma, the apparatus
comprising:
a plasma chamber defining a plasma generation area having a
circumference and a longitudinal axis;
a first RF current source;
a first antenna coupled to said first RF current source and forming
a coil about the circumference of said plasma generation area to
inductively couple RF energy into the plasma within said plasma
generation area;
a second RF current source having a phase different from the phase
of the current of said first RF current source;
a second antenna coupled to said second RF current source and
forming a coil about the circumference of said plasma generation
area to inductively couple RF energy into the plasma within said
plasma generation area;
wherein said first and second antenna coils are spaced with respect
to each other along the longitudinal axis of said plasma generation
area and electrically isolated from each other; and
wherein the phase of the current in said first antenna coil differs
from the phase of the current in said second antenna coil such that
a helicon wave with a desired wavelength in the plasma is
maintained.
19. The apparatus of claim 18 wherein said first and second current
sources comprise a generator having an output coupled to said first
antenna and a phase shifting circuit having an input coupled to
said generator output and an output coupled to said second
antennae.
20. An apparatus for launching a helicon wave in a plasma in a
semiconductor processing system, the apparatus comprising:
a chamber having a chamber wall;
a sputtering enclosure within said chamber wall and defining a
plasma containment area having a perimeter and a longitudinal axis;
said sputtering enclosure including a shield which includes first
and second coil-shields spaced with respect to each other along the
longitudinal axis of said plasma containment area and electrically
isolated from each other, each coil-shield having two ends and an
interior conductive shield wall between said two ends, each said
interior conductive shield wall being positioned as a portion of
said sputtering enclosure to define at least a portion of said
plasma containment area and to protect at least a portion of said
chamber wall from deposition materials;
a magnetic field source coupled to said chamber to establish a
magnetic field within said chamber plasma containment area;
a first RF source coupled to said ends of said first interior
shield wall of said first coil-shield, said first interior shield
wall being positioned around a perimeter of said plasma containment
area to radiate a first RF signal from said first conductive shield
wall into said plasma containment area; and
a second RF source coupled to said ends of said second interior
shield wall of second coil-shield, said second interior shield wall
being positioned around a circumference of said plasma containment
area to radiate a second RF signal from said second conductive
shield wall into said plasma containment area;
wherein the phase of said first RF signal differs from the phase of
said second RF signal such that a helicon wave in the plasma is
maintained.
21. An apparatus for launching a helicon wave in a plasma and an
axial magnetic field by coupling electromagnetic energy into the
plasma, the apparatus comprising:
a chamber having a chamber wall and a plasma containment area
within said chamber wall;
means for maintaining a helicon wave in said chamber, said helicon
wave means including:
a magnetic field source coupled to said chamber to establish a
magnetic field within said plasma containment area of said
chamber;
a first RF source for a first RF signal;
a first conductive wall means positioned within said chamber and
coupled to said first RF source, for protecting at least a portion
of said chamber wall from deposition materials, for enclosing at
least a portion of said plasma containment area and for inductively
coupling said first RF signal from said first conductive wall means
into said plasma containment area;
a second RF source for a second RF signal having a phase different
from said first RF signal; and
a second conductive wall means positioned within said chamber and
coupled to said second RF source, for protecting at least a portion
of said chamber wall from deposition materials, for enclosing at
least a portion of said plasma containment area and for inductively
coupling said second RF signal from said second conductive wall
means into said plasma containment area;
wherein the phase of said second RF signal differs from the phase
of said first RF signal such that a helicon wave with a desired
wavelength in the plasma is maintained.
Description
FIELD OF THE INVENTION
The present invention relates to plasma generators, and more
particularly, to a method and apparatus for generating a plasma in
the fabrication of semiconductor devices.
BACKGROUND OF THE INVENTION
Low pressure radio frequency (RF) generated plasmas have become
convenient sources of energetic ions and activated atoms which can
be employed in a variety of semiconductor device fabrication
processes including surface treatments, depositions, and etching
processes. For example, to deposit materials onto a semiconductor
wafer using a sputter deposition process, a plasma is produced in
the vicinity of a sputter target material which is negatively
biased. Ions created within the plasma impact the surface of the
target to dislodge, i.e., "sputter" material from the target. The
sputtered materials are then transported and deposited on the
surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths
from the target to the substrate being deposited at angles which
are oblique to the surface of the substrate. As a consequence,
materials deposited in etched trenches and holes of semiconductor
devices having trenches or holes with a high depth to width aspect
ratio, can bridge over causing undesirable cavities in the
deposition layer. To prevent such cavities, the sputtered material
can be "collimated" into substantially vertical paths between the
target and the substrate by negatively charging the substrate and
positioning appropriate vertically oriented collimating electric
fields adjacent the substrate if the sputtered material is
sufficiently ionized by the plasma. However, material sputtered by
a low density plasma often has an ionization degree of less than 1%
which is usually insufficient to avoid the formation of an
excessive number of cavities. Accordingly, it is desirable to
increase the density of the plasma to increase the ionization rate
of the sputtered material in order to decrease the formation degree
of unwanted cavities in the deposition layer. As used herein, the
term "dense plasma" is intended to refer to one that has a high
electron and ion density.
There are several known techniques for exciting a plasma with RF
fields including capacitive coupling, inductive coupling and wave
heating. In a standard inductively coupled plasma (ICP) generator,
RF current passing through a coil surrounding the plasma induces
electromagnetic currents in the plasma. These currents heat the
conducting plasma by ohmic heating, so that it is sustained in
steady state. As shown in U.S. Pat. No. 4,362,632, for example,
current through a coil is supplied by an RF generator coupled to
the coil through an impedance matching network, such that the coil
acts as the first windings of a transformer. The plasma acts as a
single turn second winding of a transformer.
This known apparatus for forming a plasma discharge suffers from
various disadvantages. In particular, power absorption in the
plasma is usually localized to a region just a few skindepths
(typically a few cm) from the outside edge of the plasma such that
the interior of the plasma generally absorbs substantially less
power than the outer edge of the plasma. As a consequence, plasma
excitation is nonuniform which may result in nonuniform etching or
deposition.
It is recognized that in a conventional Inductively Coupled Plasma
(ICP) generator using a helical coil, such as that shown in U.S.
Pat. No. 4,362,632, the electromagnetic energy radiating from each
turn of the coil antenna is in phase. Also, fields are coupled into
the plasma in a substantially pure inductive mode. The density is
usually limited to .ltoreq.10.sup.11 -10.sup.12 cm.sup.-3.
In contrast, a plasma excited using wave heating (helicon and ECR
discharges) can be excited to densities as high as several
10.sup.13 cm.sup.-3 and thus wave heating is preferred where a more
dense plasma is required. Such helicon waves are absorbed much more
uniformly throughout the discharge. Helicon waves can be excited in
a weakly magnetized (typically B<500 gauss) plasma by means of a
properly constructed antenna. In its simplest form, the helicon m=0
mode can be excited by two coil windings where the currents in each
winding are in opposite directions.
An example of a known apparatus for utilizing helicon waves to
generate plasmas of high density is shown in U.S. Pat. No.
4,990,229 to Campbell et al. U.S. Pat. No. 4,990,229 teaches that
the efficient generation of plasmas depends strongly on the antenna
configuration used. In other words, to maximize helicon wave
coupling, a very specific and sometimes complex and large antenna
configuration is often necessary. FIG. 2 of U.S. Pat. No. 4,990,229
depicts a two loop antenna used to excite the m=0 helicon mode. It
is believed that the distance between the two loops is adjusted to
match the m=0 helicon dispersion relation, i.e., ##EQU1##
where e is the charge of an electron;
.mu..sub.0 is the permittivity;
.omega..sub.c is the electron cyclotron frequency (eB.sub.0
/m.sub.e);
.omega.is the plasma frequency ##EQU2##
m.sub.e is the mass of an electron;
k.sub.z =2.pi./.lambda..sub.z is the wavenumber in axial
direction;
a is the radius of the plasma;
L is the distance between the loops;
n.sub.0 is the plasma density,
.omega.=2.pi.f is the excitation angular frequency;
B.sub.0 is the axial magnetic field; and
.epsilon..sub.0 is the permittivity of vacuum.
It is believed that for particular conditions (.omega., n.sub.0,
B.sub.0, a) the distance L between the loops of the antenna for
efficient coupling of the helicon wave is fixed by this dispersion
relation. In the approximation of k.sub.z <<3.83/a, equation
(1) can be rewritten as: ##EQU3##
for typical conditions (B.sub.0 /n.sub.0 =5.times.10.sup.-10,
f=13.6 MHz, a=15 cm) one obtains .lambda..sub.z =75 cm. This means
that the distance between the two loops is restricted to about 40
cm for efficient coupling of the m=0 helicon mode. This would lead
to a reactor aspect ratio of about unity. For large size substrates
like TFT glass or silicon wafers, this would lead to an
inconveniently large reactor volume. Also, the target to wafer
spacing would often result in being about the same as the chamber
diameter which would make it more difficult to efficiently produce
uniform films on a wafer.
Examples of other geometrically complex antenna structures required
to establish the electromagnetic fields necessary to launch the
helicon wave are illustrated in FIGS. 3 and 5 of U.S. Pat. No.
4,990,229. Such complex and often large geometries are believed
necessary in such prior art systems because most other variables
affecting helicon wavelength and coupling efficiency are fixed by
other constraints. Antenna geometry is one of the few variables
which may be somewhat more easily modified in order to establish an
appropriate electromagnetic field. For realizing both efficient
coupling of the wave energy to the electron gas, and flexible
geometry, it would be desirable to have independent control over
k.sub.z or .lambda..sub.z.
U.S. Pat. No. 5,146,137 describes various devices for the
generation of a plasma using helicon waves. These waves are
generated in one device using four or more plate-like electrodes
surrounding a quartz chamber containing the plasma. The electrodes
are coupled to a voltage source through phase shifters to produce
high frequency capacitively-coupled voltages having a phase
rotation of 90.degree.. In an alternative device, four or more
toroid-shaped coils are coupled to voltage sources to inductively
couple electromagnetic energy into the chamber. The electrodes and
coils of this reference also appear to be relatively complex.
In a number of deposition chambers such as a physical vapor
deposition chamber, the chamber walls are often formed of a
conductive metal such as stainless steel. Because of the
conductivity of the chamber walls, it is often necessary to place
the antenna coils or electrodes within the chamber itself because
the conducting chamber walls would block or substantially attenuate
the electromagnetic energy radiating from the antenna. As a result,
the coil may be directly exposed to the deposition flux and
energetic plasma particles. This is a potential source of
contamination of the film deposited on the wafer, and is
undesirable. To protect the coils, shields can be made from
nonconducting materials, such as ceramics. However, many deposition
processes involve deposition of conductive materials such as
aluminum on the electronic device being fabricated. Because the
conductive material will coat the ceramic shield, it will soon
become conducting, thus again substantially attenuating penetration
of electromagnetic radiation into the plasma.
SUMMARY OF THE PREFERRED EMBODIMENTS
It is an object of the present invention to provide an improved
method and apparatus for generating plasmas within a chamber,
obviating, for practical purposes, the above-mentioned
limitations.
These and other objects and advantages are achieved by, in
accordance with one aspect of the invention, a plasma generating
apparatus which inductively couples electromagnetic energy into the
magnetized plasma from a first antenna coil about the circumference
of a plasma generation area, and inductively couples
electromagnetic energy into the plasma from a second, separate
antenna coil about the circumference of the plasma generation area,
wherein the currents through (or voltages applied to) the first and
second coils, have a predetermined phase difference, preferably
between 1/4.pi. to 13/4.pi.. Under appropriate settings, this phase
difference in the electromagnetic fields generated by the two
antenna coils can launch a helicon wave in the magnetized plasma.
Such an arrangement has a number of advantages. For example, and as
described in greater detail below, this plasma generation apparatus
permits the antennae design for the plasma generator to be
substantially simplified and to have a substantially lower aspect
ratio. More specifically it has been determined that the chamber
shields themselves can be used as the antennae for the plasma
generator, to thereby substantially simplify the design of the
system.
In another aspect of the invention, the phase difference between
the currents in the first and second coils can be electrically
varied by, for example, a phase shifting network. As a consequence,
the chamber can be better designed to optimize factors such as
deposition efficiency, etch rate and deposition rate uniformity.
The chamber design is not limited (as is the case for many prior
art designs) by the requirement that spacing between loops in the
antenna be approximately 1/2.lambda..sub.z for a particular
antennae design. For example, the height of the chamber can be
substantially reduced even though such a reduction would affect the
spacing between the coils. By electrically varying the phase
difference between the coils, the phase difference necessary to
launch a helicon wave of a particular wavelength is readily
obtainable despite changes to the coil spacing. Thus, it is
possible to launch a wave with a half wavelength substantially
larger than the coil distance by decreasing the phase difference
substantially below .pi..
In yet another aspect of the present invention, an RF antenna for
generating a plasma in a chamber shield comprises a conductive
shield coupled in series to a coil surrounding the shield. Such an
arrangement has been found to substantially reduce attenuation of
the RF power being coupled from the outer coil, through the
conductive shield and into the chamber interior.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a plasma generating
apparatus in accordance with one embodiment of the present
invention.
FIG. 2 is a perspective, partial cross-sectional view of a PVD
chamber in accordance with an embodiment of the invention in which
two coil windings are installed within shields.
FIG. 3 is an exploded view of a PVD chamber utilizing the shields
as coil windings.
FIG. 4 is a cross-sectional view of the PVD chamber of FIG. 3.
shown installed in a vacuum chamber.
FIG. 5 is a schematic representation of a plasma generating
apparatus in accordance with another embodiment of the present
invention in which each antenna comprises a coil-shield coupled in
series with another coil.
FIG. 6 is a schematic electrical representation of an antennae of
FIG. 5.
FIG. 7 is a perspective, partial cross-sectional view of a PVD
chamber in accordance with another embodiment of the invention in
which a coil is coupled in series with a shield.
FIG. 8 is a cross-sectional view of the slot of the shield of FIG.
7 shown schematically with electrical connections.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be described hereinafter
with reference to the drawings. Referring first to FIGS. 1 and 2, a
plasma generator in accordance with a first embodiment of the
invention comprises a substantially cylindrical plasma chamber 100
in a vacuum chamber 101 in which a substantially uniform, axially
oriented magnetic field (as represented by magnetic lines of force
102) may be established through the plasma. Such a magnetic field
may be generated by, for example, Helmholtz coils (not shown)
coaxial with the chamber axis. At least two coaxial antenna coils
104 and 106 are arranged in spaced relationship around the
circumference of the chamber 100. The two antenna coils 104 and 106
may be spaced apart by a distance "L" measured along the axis of
the chamber 100. Each antenna coil comprises at least one
substantially complete turn, and each antenna coil is capable of
radiating electromagnetic energy.
The first antenna coil 102 is coupled to a first amplifier and
matching network 108. The second antenna coil 106 is coupled to a
second amplifier and matching network 112. The first and second RF
amplifiers 108 and 112 are electrically coupled to a single RF
generator 114. However, the first amplifier 108 is coupled to the
generator 114 through a phase shift regulating network 116 which
permits the current in the first antenna coil 104 and the current
in the second antenna coil 106 to be phase shifted relative to one
another.
The vacuum chamber 101 is evacuated by a pump 210 before the plasma
precursor gas is admitted into the chamber. A helicon wave may be
launched by magnetizing the plasma, and establishing an appropriate
phase difference between the first antenna coil 104 and the second
antenna coil 106 until desired conditions are met. For example, in
the embodiment shown in FIG. 2, it may be desirable to launch a
helicon wave within the chamber 100 having a wavelength .lambda.
equal to four times the distance L between the first antenna coil
104 and the second antenna coil 106. Such a helicon wave may be
efficiently launched by establishing a phase difference of .pi./2
between the currents through (or the voltage applied to) the
coils.
In general, for two coils spaced a distance L apart and a desired
wavelength of .lambda..sub.z, the phase difference .DELTA..phi. is
preferably assigned as follows:
It will therefore be seen that for a particular spacing L between
the first antenna coil 104 and the second antenna coil 106, a
plasma generator in accordance with one aspect of the present
invention allows the phase difference to be adjusted so that the
radiation emitted from the first and second antenna coils is
suitable for launching a helicon wave of wavelength .lambda.. In
other words, the phase shift regulating network 116 of plasma
generator enables the wavelength to be appropriately adjusted or
"tuned" electronically by the phase difference for any particular
geometric configuration or spacing of the antenna coils. As a
result, the plasma generator provides a greater degree of
flexibility in plasma chamber hardware design, and in particular in
the geometric design and placement of the antenna coils. In
accordance with these aspects of the present invention, launching
of an efficient helicon wave is not primarily dependent upon the
geometry and positioning of the antenna coils. Consequently, the
helicon wavelength and the plasma chamber hardware may be optimized
for other factors (such as, for example, frequency, plasma
properties, chamber size, and magnetic field) and the phase
difference may be tuned electronically to adjust the wavelength
independent of coil distance.
As shown in FIG. 2, the chamber 100 includes a shield 118
positioned between the coils 104 and 106 and the walls (not shown)
of the chamber 100 which protect the chamber walls from the
material being sputtered from a target 150 onto a semiconductor
wafer 128. However, because the coils are exposed to the deposition
flux and the energetic particles of the plasma, the coils can
become a source of contamination of the film being deposited onto
the wafer 128. To protect the coils as well as the chamber walls, a
shielding structure can be positioned between the coils and the
plasma instead of between the coils and the chamber walls. The
antennae shielding structure may have two or more cylindrical
metallic rings positioned within the plasma chamber. The portion of
the shield adjacent to the antennae may include one or more slots
to permit the electromagnetic energy radiated by the antennae to
pass through the shield to the interior of the chamber to energize
the plasma. These shields perform the function of protecting the
antenna and walls of the plasma chamber from metal deposition
during sputtering or other deposition processes.
A plasma generator in accordance with one aspect of the present
invention allows the geometry of the antenna structure to be
simplified to such an extent that the shields may themselves be
used as antenna coils, thus performing a dual function.
Furthermore, the slots previously required for energy propagation
through the shield can be largely eliminated as well, thereby
substantially simplifying the design of the shields. For example, a
shield may be formed into an antenna coil in accordance with the
present invention merely by providing a single slot or
discontinuity in the circumference of the shield, thereby
establishing a gap in what would otherwise be a closed metallic
ring. Such a cylindrical shield with a slot in the circumference
thereof forms an open loop with two distinct ends. Leads are then
attached from the amplifier and the ground strap to respective ends
of the loop, and an antenna coil is thereby formed.
Referring now to FIGS. 3-4, a plasma chamber 100A in accordance
with an alternative embodiment of the present invention is
illustrated. As shown therein, the upper coil 104A of this
embodiment has a generally cylindrical shaped-wall 119 which forms
a continuous closed metallic shield ring except for a single
vertical slot 120 which is formed between two spaced ends 121, 123
of the coil-shield 104A. The output of the RF generator 114 (FIG.
1) is coupled via the phase regulating network 116 and the matching
circuit 108 (FIG. 1) to a connection point A1 on end 121 of the
slot 120 of the coil-shield 104A. A connection point B1 at the
other end 123 of the slot 120 is connected by a ground strap 125
(FIG. 3) to ground (either directly or through a capacitor). The
coil-shield 104A acts as both an antenna to radiate RF energy from
the RF generator into the interior of the plasma chamber 100A and
also as a shield to protect the interior of the deposition chamber
from the material being deposited.
The second, lower coil-shield 106A is generally bowl-shaped and
includes a generally cylindrically shaped, vertically oriented wall
122 and a generally annular shaped floor wall 124 (FIG. 4) which
surrounds a chuck 126 which supports an item such as the wafer 128,
for example. The coil-shield 106A also has a single slot 130 which
separates two ends 132, 134 of the lower coil-shield 106. The
coil-shield 106A has two connection points, A2, B2 at the two ends
132, 134, respectively, which are coupled in a manner similar to
that of the first coil-shield 104A to the output of the
corresponding matching circuit 112 and to ground, respectively.
FIG. 4 shows the plasma chamber 100A installed in a vacuum chamber
140 of a PVD (physical vapor deposition) system. Although the
plasma generator of the present invention is described in
connection with a PVD system for illustration purposes, it should
be appreciated that a plasma generator in accordance with the
present invention is suitable for use with all other semiconductor
fabrication processes utilizing a plasma including plasma etch,
chemical vapor deposition (CVD) and various surface treatment
processes.
The vacuum chamber 140 includes a chamber wall 142 which has an
upper annular flange 144. The plasma chamber 100A is supported by
an adapter ring 146 which engages the vacuum chamber wall flange
144. The upper coil-shield wall 119 defines a surface 148 facing
the interior of the plasma chamber 100A. Sputtered deposition
material from a target 150 is deposited on the wafer 128 as
intended but is also deposited on the interior surface 148 of the
coil-shield 104A as well. The vertical wall 122 and the floor wall
124 of the lower coil-shield 106A similarly have interior surfaces
152 which also receive deposited materials sputtered from the
target 150. A clamp ring 154 clamps the wafer to the chuck and
covers the gap between the lower coil-shield 106 and the chuck 126.
Thus, it is apparent from FIG. 4 that the coil-shields 104A and
106A together with the clamp ring 154 protect the interior of the
vacuum chamber 140 from the deposition materials being deposited on
the wafer 128 in the plasma chamber 100A.
The upper coil-shield 104A has a horizontally extending outer
flange member 160 which is fastened by a plurality of fastener
screws 162 to a ceramic ring 184 resting on the adapter ring 146.
At one end 119 of the shield the coil-shield is grounded through a
short strap 161 to the adapter ring 146 (as shown in FIG. 4). The
other end (121, FIG. 2) is coupled to the RF amplifier and matching
circuit 108 through a ceramic feed through (not shown).
In the embodiment illustrated in FIG. 4, the slots 120 and 130 of
the coil-shields 104A and 106A, respectively, are at approximately
the same azimuthal angle. The cross-sectional view of FIG. 4 also
depicts the end 132 of the coil-shield 106A defining one side of
the slot 130. Accordingly, the end 132 of the coil-shield 106A
depicted in FIG. 4 provides the connection point A2 to the
coil-shield 106A. The connection point A2 includes an RF
feedthrough 170 which passes through the adapter ring 146 and is
coupled to the end 132 of the coil-shield 106A at 172 as depicted
in FIG. 4. The RF feedthrough 170 is electrically isolated from the
adapter ring 146 by an isolation tube 174 of insulative material
such as ceramic. The connection point A1 (not shown in FIG. 4)
between the RF generator 114 (via the phase shift network 116 and
the matching circuit 108, FIG. 1) to the first coil-shield 104A is
likewise constructed with an RF feedthrough similar to the RF
feedthrough 170 for the connection point A2.
The lower coil-shield 106A includes an upper flange 180 which is
fastened by a plurality of fastener screws 182 to the isolator ring
184. The isolator ring 184 isolates the lower coil-shield 106A from
the upper coil-shield 104A and also the adapter ring 146. The
adapter ring 146 has a shelf 188 which supports the isolator ring
184 which in turn supports both the upper coil-shield 104A and the
lower coil-shield 106A.
A Helmholtz coil 190 around the exterior of the vacuum chamber 140
provides the magnetic field through the plasma chamber 100A. The
target 150 is supported by an isolator ring 192 which is received
within a groove formed by a shelf 194 of the adapter ring 146 and
an upper surface of the flange member 160 of the upper coil-shield
104A. The isolator ring 192 isolates the target 150 from the
adapter ring 146 and the upper coil-shield 104A. Target, adapter
and ceramic ring 192 are provided with O-ring sealing surfaces to
provide a vacuum tight assembly from chamber flange 144 target
150.
As best seen in FIG. 4, the upper coil-shield 104A and the lower
coil-shield 106A are overlapping in the axial direction but are
spaced to define a gap 198 through which plasma precursor gases are
admitted into the interior of the plasma chamber 100A. RF energy
from the RF generator 114 (FIG. 1) is radiated from the
coil-shields 104A and 106A. The RF energy radiated by the
coil-shield 104A into the interior of the plasma chamber 100A is
phase shifted by a predetermined amount from the RF energy radiated
by the lower coil-shield 106A such that a helicon wave is launched
and maintained in the plasma chamber 100A. Because of the helicon
wave, the energy distribution of the plasma is more uniform and the
density of the plasma is increased. As a consequence, the plasma
ion flux striking the target 150 or semiconductor wafer 128 is
higher and is more uniformly distributed such that the target
material ejected from the target 150 is deposited faster and more
uniformly on the wafer material 128. The higher plasma density will
be beneficial in ionizing sputtered material from the target. As a
result, the sputtered material will be more responsive to the
collimating electric fields (not shown) adjacent to the wafer 128,
which causes the perpendicularity of the metal flux to the wafer
128 to be significantly enhanced. Consequently, fine features may
be coated more uniformly, and high aspect ratio holes and trenches
may be filled with little or no void formation. Collimating
electric fields may be induced by electrically biasing the wafer
and/or pedestal negatively with respect to the plasma with an RF
supply 151 (FIG. 1) to impose an HF RF signal (e.g., 13.6 Hz) to
the pedestal through a matching network. These techniques are known
to those skilled in the art.
In the embodiment illustrated in FIG. 3, a magnet structure 1001 is
located behind the target. This magnet serves to determine the
erosion profile on the target and can be optimized to give uniform
film thickness or the wafer 128. The magnet structure 1001 may have
one of several configurations designed to provide a desired erosion
profile on the target. In each instance, the structure 1001 may
include one or more magnets which are moved across the back side of
the target during sputtering. It should be realized that if no RF
power is applied to the coil-shields and only DC negative bias to
the target 150, the chamber 100A closely resembles a conventional
PVD chamber such as those currently installed on Endura PVD systems
manufactured by Applied Materials, Inc., the assignee of the
present application.
It is also noted that in the embodiment illustrated in FIG. 1, only
a single phase shift regulating network is illustrated in
combination with a pair of antennas and a single RF generator. In
alternative embodiments of the present invention more than one
phase shifter may be used, and correspondingly more than two
antenna coils and associated amplifiers may also be used.
It is seen from the above that a plasma generator in accordance
with one preferred embodiment of the present invention can
substantially simplify the design of a deposition or other
processing chamber which generates high density plasmas. By
utilizing the shields of the chamber as the antenna coils of the RF
generator, the need for a separate antenna structure and associated
isolator members can be eliminated.
Still further, because the requirements for a particular size and
shape for the antenna can be substantially relaxed, the chamber can
be very compact. As shown in FIG. 4, the upper coil-shield 104 and
the lower coil-shield 106 are very closely spaced and even overlap
in the axial direction. Notwithstanding this very close spacing, by
properly selecting the phase difference between the currents
generated in the coil-shields 104 and 106, a helicon wave may be
launched leaving a wavelength .lambda. as determined by the
frequency of the RF generator 114 and the phase difference.
In the illustrated embodiment, the chamber wall 142 has a width
(measured in the radial direction) of 16" but it is anticipated
that good results can be obtained with a width in the range of
6"-25". The wafer to target space is preferably about 2" but can
range from about 1.5" to 8". The frequency of the generator 114 is
preferably 13.6 MHz but it is anticipated that the range can vary
from, for example, 1 MHz to 100 MHz. A variety of precursor gases
may be utilized to generate the plasma including Ar, H.sub.2,
O.sub.2 or reactive gases such as NF.sub.3, CF.sub.4 and many
others. Various precursor gas pressures are suitable including
pressures of 0.1-50 mT. For ionized PVD, a pressure around 10-20 mT
is preferred for best ionization of sputtered material. Similarly,
the strength of the magnetic field may vary from 20 to 1000 gauss
but a field strength of about 200-500 gauss is preferred. The phase
shift should be adjusted to optimize helicon wave coupling but
generally is in the range of 1/4.pi. to 1 3/4.pi. for optimum
performance.
The coil-shields may be fabricated from a variety of conductive
materials including aluminum and stainless steel. Although the
slots 120 and 130 are shown as being approximately aligned at the
same azimuthal angle in FIGS. 3 and 6, the slots of the
coil-shields need not be aligned but may be at any angle relative
to each other as indicated in FIG. 3.
The coils 104, 106 and 104a and 106a of the illustrated embodiments
described to this point are each depicted as a single turn coil.
However, it should be appreciated that each coil may be implemented
with multiple turn coils. Because the flux induced by coils is
proportional to the square of the number of turns of the coil, it
may be advantageous to increase the number of turns of the coil. In
accordance with yet another aspect of the present invention, a
coil-shield may be coupled in series with a helical coil such that
the coil-shield is one turn of an RF antenna coil having a
plurality of turns.
FIG. 5 shows a plasma generator in accordance with another
embodiment of the present invention which is similar to the plasma
generator of FIG. 1 except that the coaxial antenna coils 204 and
206 each comprise a coil-shield electrically connected in series to
a multi-turn helical coil which surrounds the associated
coil-shield. This arrangement may be more readily understood by
reference to FIG. 6 which shows a schematic representation of the
electrical connections of the coil 204 which includes a coil-shield
204a which is connected to a multi-turn helical coil 204b. One end
of the helical coil 204b is coupled to an RF source such as the
output of the first amplifier and matching network 108 (FIG. 5),
the input of which is coupled to the RF generator 114 through the
phase shift regulating network 116. The coil-shield 204a, like the
coil-shield 104 of FIG. 3, has a slot 220 which defines two ends
221 and 223. The helical coil 204b is connected to the coil-shield
204a at end 223 of the coil-shield coil 204a . The other end 221 of
the coil-shield 204a is coupled to ground, preferably through a
capacitor. As best seen in FIG. 5, the helical coil 204b when
installed is positioned to surround the coil-shield 204a. The turns
of the helical coil 204b are wound around but insulatively spaced
from the coil-shield 204a so that the current circulating through
the helical coil 204b travels in the same circular direction as the
current through the coil-shield 204a. Consequently, the magnetic
fields induced by the helical coil 204b are in phase with the
magnetic fields induced by the coil-shield 204a. The second coil
206 is similarly constructed of a coil-shield 206a coupled in
series with a helical coil 206b which surrounds the coil-shield
206b. The turns of the helical coil 206b are likewise wound in
phase with the turn of the coil-shield 206a.
Such an arrangement has been found to have a number of advantages.
For example, it has been found that the RF power emanated by the
helical coils are effectively coupled into the chamber through the
associated coil-shields into the interior of the chamber. Any
attenuation caused by the coil-shields is substantially reduced. At
the same time, the coil-shields effectively protect the helical
coils and other portions of the interior of the chamber from being
coated or damaged by the various semiconductor processes including
sputtering.
In addition, because each coil has a plurality of turns, the
necessary power to produce a desired flux level in the chamber
interior can be substantially reduced as compared to a single turn
coil. High power levels may not be appropriate for some
applications because of, for example, the added stress to
components which can necessitate using components having a higher
current carrying capacity.
An RF antenna comprising a series coupled coil-shield and coil in
accordance with one aspect of the present invention may be used in
semiconductor processing apparatus other than those requiring the
launching of a helical wave in a high density plasma. For example,
FIG. 7 illustrates a chamber 400 utilizing just one such RF antenna
which has been found to generate a satisfactory high density plasma
without the use of helicon waves.
The RF antenna for generating the high density comprises a
coil-shield 304a which is electrically coupled in series with a
helical coil 304b which surrounds the coil-shield 304a. The
coil-shield 304a is very similar to the coil-shield 104 of FIG. 4
except that the coil-shield 304a extends further down to a position
below the top of the wafer (not shown) because in this embodiment,
the chamber has only the one coil-shield, that is, coil-shield
304a. At the bottom of the coil-shield 304a is a horizontal annular
lip 410 which terminates short of the clamp ring 154. Instead of a
second coil-shield found in the earlier embodiments for launching
helicon waves, the embodiment of FIG. 7 has a generally annular
shaped grounded lower shield 420 which protects the chamber between
the clamp ring 154 and the annular lip 410 of the coil-shield 304a.
The lower shield 420 is spaced from the coil-shield 304a and is
grounded to the chamber ground.
In the illustrated embodiment, the helical coil 304b is formed of a
ribbon shaped copper wire which is wound in three helical turns
surrounding the coil-shield 304a. The helical coil 304b is
supported between an inner ceramic member 430 and an outer ceramic
member 432 of a ceramic assembly 434. The ceramic assembly 434
insulates the helical coil 304b from the chamber and also from the
coil-shield 304a. The lower shield 410 has a lip 440 which is
received by the outer ceramic member 432 which supports the lower
shield 420.
In the embodiment of FIG. 7, the slot 450 separating the two ends
of the coil-shield 304a is covered by a cover member 452 which is
spaced from the coil-shield 304a by insulative ceramic standoffs
454 as best seen in FIG. 7. The cover member 452 shields the slot
450 from the material being sputtered. It is important to prevent
sputtered material from passing through the slot or bridging across
the slot to form a conductive path which could short the two ends
of the slot together. The slots of the shields of the earlier
described embodiments preferably have a similar cover member,
either in front of or behind the associated slot. One end 461 of
the coil-shield 304a is coupled to ground by a capacitor 464. The
other end 463 of the coil-shield 304a is coupled to one end of the
helical coil 304b as previously described. It is important that the
coil-shield 304a be electrically coupled to the helical coil 304b
in such a manner that current passing through the coil-shield 304a
travels in the same circular direction as the current traveling
through the helical coil 304b so that the magnetic fields of the
coil-shield 304a and helical coil 304b are in phase.
The chamber 400 of the embodiment of FIG. 7 further includes a
source adaptor member 470 which is coupled to chamber ground. A DC
return ring 472 abuts the source adaptor ring 470 and is also
coupled to ground. The coil-shield 304a is supported by the inner
ceramic member 432 of the ceramic assembly 434 and is insulated
from the DC return shield 472 and the source adaptor 470 by an
insulating ring 474. As shown in FIG. 7, the coil-shield 304a is
spaced from all conductive components to prevent undesirable arcing
since the coil-shield is a part of the RF antenna emanating high RF
energy to generate a high density plasma.
In another advantage of utilizing a coil-shield as part or all of
the RF antenna for generating a plasma, it is believed that the RF
potential applied to the coil-shield is capacitively coupled to the
precursor gas to assist in initiating the generation of the plasma.
However, it is recognized that the RF potential can also cause the
coil-shield itself to be sputtered in addition to the target of the
chamber. Accordingly, in order to prevent contamination of the
wafer from material being sputtered from the coil-shield, it is
preferred that the chamber be preconditioned by initiating
sputtering of the target without the application of RF energy to
the coil-shield and before the wafer is brought into the chamber
for processing. In this manner, the target material can be
sputtered and deposited onto the coil-shield to a sufficient
thickness to prevent the underlying material of the coil-shield
from being sputtered when the wafer is in the chamber.
Alternatively, if a target material is made of a conductive
material and only one type of material is to be sputtered, the
coil-shield may be manufactured from the same material as the
sputtered target.
In another aspect of the present invention, it has been recognized
that sputtering of the coil-shield can also be reduced by choosing
the circuit components such that a series resonance point is
created at or near the center line 480 of the vertical wall of the
coil-shield 304a. This resonance condition is preferably achieved
by adjusting the capacitance of the capacitor 464 (FIG. 8) coupling
one end 461 of the coil-shield 304a to ground. In a preferred
embodiment, the capacitance of the capacitor 464 is empirically
determined by measuring the voltages at the top and bottom of the
wall of the coil-shield 304a while the capacitance of capacitor 464
is adjusted. Once the voltages at the top and bottom of the
coil-shield 304a are substantially equal in magnitude but
180.degree. out of phase, a resonance point, i.e., a point of
minimum voltage potential will be created at the center 180 of the
wall of the coil-shield 304a such that the center 480 will be
maintainable at an RF ground. Such an arrangement minimizes the
magnitudes of the voltages applied to the coil-shield 304a which is
believed to correspondingly reduce sputtering of the coil-shield.
For example, for an antennae having an inductance of approximate
4-5 micro henries at an RF frequency of approximately 4 Megahertz,
a capacitance of approximately 0.025 micro farads is believed to be
suitable. These values would of course vary, depending upon the
particular geometries of the various components.
The coil-shield is preferably made of a highly conductive material
such as stainless steel unless made of the same material as the
sputtered target material as noted above. Other materials may also
be used. The coil-shield material should however be a highly
conductive material and one having a coefficient of thermal
expansion which closely matches that of the material being
sputtered to reduce flaking of sputtered material from the
coil-shield onto the wafer.
In addition, for purposes of simplicity, the coil-shield 304a has
been illustrated as a wall member having a generally flat annular
shape except at the top and bottom sides of the shield wall.
However, because of the relatively low aspect ratio of the
coil-shield 304a and the helical coil 304b, it is anticipated that
the magnetic field lines adjacent to the coil-shield 304a may have
a curvature. Accordingly, it is anticipated that loss producing
eddy currents in the coil-shield may be reduced and the performance
of the system thereby improved by curving the wall of the
coil-shield 304a to have a generally concave (i.e. inward curving)
cross-section to more closely match the curvature of the field
lines.
More specifically, the magnetic field is created by the current
passing through the turns of the coil including the coil-shield.
The total magnetic field at a particular point in the interior of
the chamber is a function of the coil geometry including the aspect
ratio (height to width) of the coil and the spacings of the coil
turns. For a perfect solenoid, the magnetic field would be parallel
to the center axis of the coil. However, because of the low aspect
ratio of the coil of the illustrated embodiments, it is anticipated
that the magnetic field lines may be somewhat curved adjacent to
the shield-coil. Magnet field lines which intersect the conductive
shield will produce eddy surface currents which in turn induce
magnetic fields opposing to the intersecting field to in effect
cancel at least a portion of the magnetic field intersecting the
shield. Because the conductive shield has a resistance, the eddy
currents consume power which produces losses.
This RF magnetic field induced in the chamber containing the
precursor gas excites free electrons which collide with the atoms
of the precursor gas to ionize the precursor gas. Electrons freed
from the ionized precursor gas continue to collide with other atoms
of the precursor gas setting up an avalanche condition which
rapidly ionizes the precursor to create a dense plasma of free
electrons and ionized gas.
The neutral atoms of the sputtered material which subsequently pass
through the plasma are struck by the excited free electrons which
ionize the sputtered material. As discussed above, it is desired to
ionize as much of the sputtered material as possible to facilitate
collimating the sputtered material. To efficiently generate the
magnetic fields, losses due to eddy currents should be minimized.
Hence, it is preferred to curve the coil-shields as appropriate to
match the curvature if any of the magnetic field lines to reduce
undesirable eddy current losses.
The chamber 400 may be fabricated of materials and dimensions
similar to those described above in connection with other
embodiments, modified as appropriate for the particular
application. The coil 304b of the illustrated embodiment is made of
3/8 by 1/8 inch heavy duty copper ribbon formed into a three turn
helical coil. However, other highly conductive materials and shapes
may be utilized. For example, hollow copper tubing may be utilized,
particularly if water cooling is desired. The RF generators 114,
matching circuits 108 and 112, phase regulating network 116 and
adjustable capacitor 464 are components well known to those skilled
in the art. For example, an RF generator such as the ENI Genesis
series which has the capability to "frequency hunt" for the best
frequency match with the matching circuit and antenna is
suitable.
It will, of course, be understood that modifications of the present
invention, in its various aspects, will be apparent to those
skilled in the art, some being apparent only after study others
being matters of routine mechanical and electronic design. Other
embodiments are also possible, their specific designs depending
upon the particular application. As such, the scope of the
invention should not be limited by the particular embodiments
herein described but should be defined only by the appended claims
and equivalents thereof.
* * * * *